1. Technical Field
The present invention relates to integrated circuits, and more particularly to three-dimensional integrated circuit transformer structures configured for variable turns ratios for use with high frequency applications.
2. Description of the Related Art
With an increased demand for personal mobile communications, integrated semiconductor devices such as complementary metal oxide semiconductor (CMOS) devices may, for example, include voltage controlled oscillators (VCO), low noise amplifiers (LNA), tuned radio receiver circuits, or power amplifiers (PA). Each of these tuned radio receiver circuits, VCO, LNA, and PA circuits may, however, require on-chip inductor components in their circuit designs.
Several design considerations associated with forming on-chip inductor components may, for example, include quality factor (i.e., Q-factor), self-resonance frequency (fSR), and cost considerations impacted by the area occupied by the formed on-chip inductor. Accordingly, for example, a CMOS radio frequency (RF) circuit design may benefit from, among other things, one or more on-chip inductors having a high Q-factor, a small occupied chip area, and a high fSR value. The self-resonance frequency (fSR) of an inductor may be given by the following equation:
            f      SR        =          1              2        ⁢        π        ⁢                  LC                      ,where L is the inductance value of the inductor and C may be the capacitance value associated with the inductor coil's inter-winding capacitance, the inductor coil's interlayer capacitance, and the inductor coil's ground plane (i.e., chip substrate) to coil capacitance. From the above relationship, a reduction in capacitance C may desirably increase the self-resonance frequency (fSR) of an inductor. One method of reducing the coil's ground plane to coil capacitance (i.e., metal to substrate capacitance) and, therefore, C value, is by using a high-resistivity semiconductor substrate such as a silicon-on-insulator (SOI) substrate. By having a high resistivity substrate (e.g., >50 Ω-cm), the effect of the coil's metal (i.e., coil tracks) to substrate capacitance is diminished, which in turn may increase the self-resonance frequency (fSR) of the inductor.
The Q-factor of an inductor may be given by the equation:
      Q    =                  ω        ⁢                                  ⁢        L            R        ,where ω is the angular frequency, L is the inductance value of the inductor, and R is the resistance of the coil. As deduced from the above relationship, a reduction in coil resistance may lead to a desirable increase in the inductor's Q-factor. For example, in an on-chip inductor, by increasing the turn-width (i.e., coil track width) of the coil, R may be reduced in favor of increasing the inductors Q-factor to a desired value. In radio communication applications, the Q-factor value is set to the operating frequency of the communication circuit. For example, if a radio receiver is required to operate at 2 GHz, the performance of the receiver circuit may be optimized by designing the inductor to have a peak Q frequency value of about 2 GHz. The self-resonance frequency (fSR) and Q-factor of an inductor are directly related in the sense that by increasing fSR, peak Q is also increased.
On-chip transformers are formed from inductor-like structures. On-chip transformers are needed in radio frequency (RF) circuits for a number of functions including impedance transformation, differential to single conversion and vice versa (balun), DC isolation and bandwidth enhancement to name a few. Some performance metrics of on-chip transformers may include a coefficient of coupling (K), occupied area, impedance transformation factor (turns ratio), power gain, insertion loss, efficiency and power handling capability.